Chapter 28 - Development

SPERM MIGRATION In upper 1/3 of fallopian tubes within 24 hrs of ovulation & 48 hrs of insemination. Only about 3000 of original 300 million make it.

CAPACITATION Even after arrival at egg, sperm can’t fertilize. Fluids in female soften plasma membrane and dilute inhibitory factors that prevent the acrosome from working Ca++ diffuses in and enhances tail lashing

CAPACITATION Timing – sperm live only 6 days, so can’t get pregnant more than a week before ovulation, nor more than 14 hours after [ egg won’t be viable long enough]

Fertilization Several sperm needed to enzymatically penetrate cell layers around the ovum – acrosome reaction – exocytosis of acrosome Only 1 sperm enters the ovum - tail & midpiece disintegrate, egg releases enzymes to destroy sperm surface receptors – prevents polyspermy.

Fertilization

MEIOSIS Oocyte now completes meiosis. 23 male chromosomes + 23 female chromosomes join to form zygote.

TRIMESTERS 1 - Fertilization through week 12 – most sensitive time-over half die 2 - Weeks 13 through 24 – complete most organ development – near end IS possible to survive birth  

TRIMESTERS 3 - 25 weeks – birth - growth continues to point where survival is more likely – brain liver & kidneys have to develop further AFTER birth. Single births usually 40 weeks, twins 35 weeks 

CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT Mitotic cell divisions produce identical copies. Morula - solid ball of cells. Blastocyst - hollow ball - cavity is called blastocoel. Made of trophoblast [outer layer] & Inner cell mass

Human Morula

Human Blastocyst

CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT Becomes implanted in endometrium - inner cell mass facing endometrial wall. TAKES ABOUT 1 WEEK. Trophoblast gives rise to chorion - forms placenta with chorionic villi – secretes hCG to maintain corpus luteum.

CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT Inner cell mass forms embryo and other extra-embryonic membranes

IMPLANTATION ~ 6 days after ovulation Trophoblast cells facing wall fuse  syncytiotrophoblast – grows into uterus Uterus responds by growing over and burying entire blastocyst Ideally blastocyst implants HIGH on the wall. Trophoblast grow into placenta

IMPLANTATION ~ 6 days after ovulation Placenta - nutritive bridge between mother & fetus –by end of 3rd month Produces large amounts of hCG - peaks at wk 8-9, drops to constant level at wk 16, stimulates corpus luteum to continue secretion of ES and P Later, the placenta secretes its own P and ES to maintain pregnancy, and relaxin to aid in delivery and prevent premature contractions

EXTRA-EMBRYONIC MEMBRANES Chorion - outer -derived from trophoblast of blastocyst. Forms placenta. Secretes estrogens, progesterone, relaxin. Amnion - for protection. Filled with amniotic fluid. Shock absorber for fetus. Repository for fetal urine, etc.

EXTRA-EMBRYONIC MEMBRANES Allantois - blood vessels to placenta and umbilicus. Later becomes ligament attached to urinary bladder. Yolk sac - has little yolk, but source of early blood cells & primordial gametes.

EMBRYOGENESIS Embryoblast flattens into embryonic disc in amniotic cavity – 2 layers – epiblast [toward cavity, hypoblast – away from] Primitive streak forms along midline with primitive groove running down it  bilateral symmetry, front and back defined

EMBRYONIC DEVELOPMENT - Gastrulation Results in 3 germ layers - ectoderm, endoderm, mesoderm. Epiblast will become ectoderm [skin, etc. and CNS] Migration of epiblast cells into and through the primitive groove to replace hypoblast cells with endoderm – will be gut lining

EMBRYONIC DEVELOPMENT - Gastrulation Then a third layer migrates in between the two = mesoderm [will become mesenchyme – basis of connective tissues, and muscle